(1) Field of the Invention
The invention relates to high-speed underwater vehicles and is directed more particularly to supercavitating vehicles that move in a cushion of air underwater, and to an on-board assembly and method for measurement of the speed of the vehicle through a water environment.
(2) Description of the Prior Art
Recent investigations into high speed underwater vehicles have focused attention on providing vehicles that ride in a cushion of air to achieve high speeds in water. For a nominal prior art streamlined, fully wetted underwater vehicle, 70% of the overall drag is skin friction drag, the remainder is pressure or blockage drag. Supercavitation allows for much higher speeds to be sustainable by eliminating, or at least substantially reducing, skin friction drag. The conditions for supercavitation require that enough energy be put into the water to vaporize a given volume of water through which the vehicle travels. This is done by accelerating fluid over a sharp edge, usually the nose of a vehicle, such as a torpedo, so that the pressure drops below the vapor pressure of water. If the speed of the object is not fast enough to travel through the vapor cavity before the cavity collapses, artificial ventilation into the cavity can keep the cavity “open” until the object moves past. When a cavity completely encapsulates an object, by vaporous and/or vented cavitation, it is referred to as “supercavitation”. The vehicle nose, or “cavitator”, and aft control surfaces, are the only parts of the vehicle in constant contact with the water through which the vehicle travels. The cavity closure typically is positioned just forward of the aft control surfaces.
A supercavitating vehicle achieves high speed by minimizing the wetted contact area. The entire vehicle, with the exception of the cavitator and the aft control surfaces, is not in contact with the water through which the vehicle moves. The optimal use of this small wetted contact area presents problems with respect to cavity formation means and vehicle control systems, all of which must be disposed in this limited space. Traditional homing systems require surface space in order to function. However, in supercavitating vehicles, the use of the cavitator surface as a velocity sensor is impractical. Further, the lack of wetted contact areas means traditional methods of using dynamic fluid pressure for determining speed are not practical. In as much as supercavitating vehicle technology is in its early stages of development, a complete understanding of vehicle propulsion and hydrodynamics is insufficient to determine vehicle velocity from on-board sensing of fuel consumption, ventilation gas flow rates, and the like.
Accordingly, there is a need for a sensor assembly and method for determining the speed of a supercavitating underwater vehicle, as the vehicle travels through water. There is further a need for such an assembly as can be carried on-board the vehicle.
There is similarly a need for a method for determining the speed of the vehicle through water, using on-board sensors.